This application is based upon Japanese Patent Application Nos. Hei. 11-84284 filed on Mar. 26, 1999, and 2000-25262 filed on Feb. 2, 2000, the contents of which are incorporated herein by reference.
1. Field of the Invention
This invention relates to infrared rays detection apparatuses, and particular to an infrared rays detection apparatus, which has an optical window (e.g., a condenser lens) having a characteristic that scatters noise right such as visible right.
2. Related Art
Recently, an infrared rays detection apparatus is used for a vehicular control, such as an air-conditioner control or an air bag control, by detecting a position of passenger in a vehicle. Furthermore, this kind of infrared rays detection apparatus is also used for a security control by detecting an invasion into the vehicle during vehicle""s parking. That is, various vehicle controls are performed by image-forming infrared rays radiated from a person body on a thermal detection element by using a condenser lens, and by detecting an existence of a person based on a detection output from the thermal detection element, and by performing the various controls based on the detection result.
Here, a wavelength of the infrared rays radiated from a person body is approximately within a wavelength range of 8-12 xcexcm. Since infrared rays have such the wavelength range, the infrared rays may be declined when a normal condenser lens is used. Therefore, it is preferable to employ a condenser lens made of a particular material such as a high-density polyethylene, a chalcogens glass, BaF2, or ZnS.
However, since such the particular material indicates great light permeability against light having a wide frequency, noise light such as solar light may enter the thermal detection element through the condenser lens, so that the security control may falsely judge the existence of person.
On the contrary, in JP-A-61-39001 and JP-A-9-21701 disclose the following technology. That is, particulates having different refractive indexes or bubbles are mixed into an optical window member for infrared rays, so that particular infrared rays within a specific wavelength band, which is a target wavelength band to be detected by the thermal detection element, can be transmitted, and that visible right or near infrared rays can be scattered. As a result, an S/N ratio (signal/noise ratio) of a thermal detection element can be improved
However, even in an infrared rays sensor including the optical window member having this characteristic, when noise light such as reflected solar light is entered, the noise light may influence a detection output of the infrared rays sensor. That is, when the optical window member is positioned close to the thermal detection element, noise light scattered by the optical window member may enter the thermal detection element. In this case, the detection output of the thermal detection element may indicate twice as much as a normal detection output due to the noise light entered to the thermal detection element. In this case, the security control may falsely detect a target object to be measured, when the security control cannot judge whether an output change of the thermal detection element is due to the noise light or a change of the target object to be measured itself.
Here, it can be thought that an additional sensor such as an illuminometer to measure an amount of entered light is added to the infrared rays sensor, so as to judge an entering of the noise light. However, this countermeasure may become bulky. Furthermore, it can be thought that a filter for filtering off the visible light is applied. However, this countermeasure is also not preferable because the filter may also decrease a transparency of the wavelength band to be originally needed.
This invention has been conceived in view of the background thus far described and its first object is to reduce an influence of noise light with a simple structure.
Its second object is to provide an infrared rays detection apparatus, which has an optical window having a characteristic that scatters noise light, being capable of reducing an influence of noise light with a simple structure.
According to the present invention, infrared rays radiated from a target object to be measured is condensed by an optical window, and is image-formed on a plurality of thermal detection elements. A thermal data generator generates a thermal image of the target object to be measured based on the detection outputs from the thermal detection elements. Here, when noise light is entered to the optical window, the noise light is scattered by the optical window; however, the scattered noise light may be entered to the thermal detection elements. In this case, the detection outputs from almost all the thermal detection elements uniformly change, because such scattered noise light is uniformly entered to the almost all the thermal detection elements. Therefore, when the detection outputs uniformly change each other, a judging portion determines that the noise light is entered to the thermal detection elements. Hence, it can prevent from decreasing of reliability of making the thermal image data by the thermal image data generator. That is, it can reduce an influence of noise light with a simple structure.
According to another aspect of the present invention, infrared rays radiated from a target object to be measured is condensed by an optical window, and is image-formed on a plurality of thermal detection elements. A thermal data generator generates a thermal image of the target object to be measured based on the first detection output from the thermal detection element. Here, when noise light is entered to the optical window, the noise light is scattered by the optical window; however, the scattered noise light may be entered to a reference element aw well as the thermal detection element. Therefore, an output portion calculates a difference between the first detection output and the second detection output, and outputs the difference as an output indicating the infrared rays. Hence, it can prevent from decreasing of reliability of making the thermal image data by the thermal image data generator. That is, it can reduce an influence of noise light with a simple structure.